API: Ignore add_bos_token in chat completions

When fetching special tokens from the model, don't factor in the
add_bos_token and ban_eos_token parameters as switches.

In addition, change the internal handling of add_bos_token to an optional
boolean. This allows us to fallback to the model when selecting whether
or not to add the BOS token, especially for chat completions.

Signed-off-by: kingbri <8082010+kingbri1@users.noreply.github.com>

endpoints/OAI/types/chat_completion.py

@@ -1,4 +1,4 @@
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, field_validator
 from pydantic.json_schema import SkipJsonSchema
 from time import time
 from typing import Literal, Union, List, Optional, Dict
@@ -82,6 +82,12 @@ class ChatCompletionRequest(CommonCompletionRequest):
     tool_call_end: SkipJsonSchema[Optional[str]] = None
     tool_call_schema: SkipJsonSchema[Optional[dict]] = tool_call_schema
 
+    @field_validator("add_bos_token", mode="after")
+    def force_bos_token(cls, v):
+        """Always disable add_bos_token with chat completions."""
+
+        return None
+
 
 class ChatCompletionResponse(BaseModel):
     id: str = Field(default_factory=lambda: f"chatcmpl-{uuid4().hex}")